Numerous systems exist in the prior art for the liquefaction of a hydrocarbon feed stream by heat exchange with one or more refrigerants such as propane, propylene, ethane, ethylene, methane, nitrogen or combinations of the preceding refrigerants which are referred to in the art as “mixed refrigerant” systems. Examples of liquefaction processes using mixed refrigerants are given in U.S. Pat. No. 5,832,745, U.S. Pat. No. 6,389,844, U.S. Pat. No. 6,370,910 and U.S. Pat. No. 7,219,512 (the contents of which are hereby specifically incorporated by reference). As methods and systems for liquefying a hydrocarbon stream are well known in the art, they do not form a portion of the present invention and thus the operating conditions of the refrigeration side and the compositions of the refrigerants are not discussed in detail here.
A typical mixed refrigerant stream may is nominally 50% ethane, 25% propane, 25% methane and 1-5% nitrogen depending on the operating temperature of the main cryogenic heat exchanger. The methane and the nitrogen are used to cool the top of the cold tube bundle. The ethane provides the majority of the cooling that takes place in the middle of the tube bundles, with the propane providing the cooling duty for the lower portion of the warm bundle at the bottom of the main cryogenic heat exchanger. During normal LNG production operations, the mixed refrigerant circulates between the main cryogenic heat exchanger and a mixed refrigerant compression circuit. At first start-up of an “empty” LNG plant e.g. at a new or “greenfield” site, there is no mixed refrigerant available at site. Propane can be readily purchased and imported to a greenfield site but this is not the case for ethane.
The traditional way of producing ethane for the start-up of an LNG production plant is fill up the propane circuit with imported propane and then run a natural gas feed stream through a scrub column to extract ethane by providing cooling to the top of the scrub column by operating the propane circuit. The natural gas feed stream is run through the scrub column at a reduced rate of between 30 and 40% of the normal operating flow rate for the natural gas feed stream that would be used if the plant was producing LNG. The liquids that drop out in the scrub column are delivered to a fractionation facility including a deethaniser to recover ethane that is stored in a sphere until sufficient ethane has been recovered to supply the required amount of ethane needed for the mixed refrigerant inventory of the LNG plant. Using this prior art process, several weeks of operation may be required to produce a sufficient inventory of ethane for start-up because the extraction efficiency of the scrub column for ethane is around 5%. During this period of time, significant quantities of the gas are flared. In addition to this, running the pipeline or trunkline that delivers a wet natural gas feed stream to the LNG production plant at low velocities causes significant liquid management issues. Compounding the problem, the load on the propane compression circuit is low, requiring the use of recycle valves to keep the propane compressors operational. The recycle stream is warmer than ambient temperature, reducing the efficiency of propane compression. Whilst this prior art process is used for natural gas feed streams that are rich in ethane, an alternative process is needed to handle natural gas feed streams that are lean.
It has been suggested to attempt to start-up an LNG production plant using a mixture of propane and methane without ethane at all. However, this prior art process can only work if the main heat exchanger is capable of being operated at low flow rates in the order of 10 to 15% of the normal LNG production design flow rate. Under normal LNG production operating conditions, natural gas is fed into the bottom of a plurality of vertically oriented tubes within the shell of the main heat exchanger with the liquefied gas that is drawn out of the main heat exchanger passing vertically up the tubes. When an attempt is made to operate the main heat exchanger at a low rate, there is insufficient flowing pressure drop across the tubes of the main heat exchanger to force the liquid out of the top of the tubes. Consequently, when operated at low flow rates, there is a risk of the liquefied gas flowing backwards down the tubes under the influence of gravity. When this occurs, the majority of the tubes fill up with liquid whereby the flowing pressure drop in the remaining tubes is sufficient to force the liquid out of the top. The temperature profile becomes unstable with resulting increases in the mechanical stresses on the main heat exchanger vessel. It is unlikely that production in excess of 50% of design can be achieved with such a mixture of refrigerants.
It is also known to import ethylene in isotainers as a substitute for ethane. However, ethylene has different blast properties to ethane which can result in safety issues unless the plant is specifically designed to run on ethylene from the outset. Using ethylene requires the use of higher separation distances between equipment items requiring a change of layout, a less compact footprint, and consequently an additional cost to construction for a “one-off” usage at start-up.
There remains a need for an alternative method for the production of ethane for starting up an LNG production plant.